Question

In cucumbers, dull fruit (D) is dominant over glossy (d) fruit, orange friut (R) is dominant over cream fruit (r), and bitter cotyledon (B) is dominant over non-bitter cotyledons (b). The three characters are encoded by genes located on different pairs of chromosomes. A plant homozygous for dull, orange fruit and bitter cotyledons is crossed with a plant that is homozygous for glossy, cream fruit and non-bitter cotyledons. The F1 are intercrossed to produce the F2. Assume that all 3 traits are encoded by independent genes.

a. Write out the parental anf F1 crosses using appropriate genotypic annotation. Indicate the phenotypes and their expected proportions in the F1 and F2.

b. An F1 plant is crossed with a plant that has glossy, cream fruit and non-bitter cotyledons. Write out this cross using appropriate annotation, and give the genotypes, phenotypes, and respectuve proprtions among the progeny of this cross.

Answer 1

In the cross of cucumbers with different traits, the F1 generation will be heterozygous with dominant phenotypes, and a 9:3:3:1 phenotypic ratio is expected for the F2 generation. A test cross between an F1 plant and a homozygous recessive plant will result in a 1:1:1:1 phenotypic ratio among the progeny.

Step-by-step explanation:

In cucumbers, dull fruit (D) is dominant over glossy (d) fruit, orange fruit (R) is dominant over cream fruit (r), and bitter cotyledon (B) is dominant over non-bitter cotyledons (b). These genes are on different chromosome pairs, hence they assort independently.

Parental and F1 Cross

The parental (P) generation cross would be: DDRRBB (dull, orange, bitter) x ddrrbb (glossy, cream, non-bitter). The F1 generation will all be heterozygous DdRrBb, displaying the dominant phenotypes: dull, orange, and bitter cotyledons. Since all traits are dominant in the F1, the expected proportion of the dominant phenotype in F1 is 100%.

F2 Generation

When F1 individuals are intercrossed (DdRrBb x DdRrBb), the expected phenotypic ratio for F2 is 9:3:3:1 for each trait combination due to independent assortment. Hence, we anticipate 9/16 dull and orange, 3/16 dull and cream, 3/16 glossy and orange, and 1/16 glossy and cream for the fruit traits. A similar breakdown can be applied for cotyledon bitterness, with 3/4 being bitter and 1/4 non-bitter.

Test Cross with F1

When an F1 plant (DdRrBb) is crossed with a homozygous recessive plant (ddrrbb), the expected genotypic ratio among the progeny will be 1:1:1:1 for each trait, and the phenotypic ratio will appear as 1 dull and orange : 1 dull and cream : 1 glossy and orange : 1 glossy and cream, with a similar breakdown for cotyledon bitterness.

Answer 2

Step-by-step explanation:

• Dull fruit (D) is dominant over glossy (d) fruit.
• Orange friut (R) is dominant over cream fruit (r).
• Bitter cotyledon (B) is dominant over non-bitter cotyledons (b).

The three genes are independent.

a)

• A plant homozygous for dull, orange fruit and bitter cotyledons has the genotype DDRRBB.
• A plant homozygous for glossy, cream fruit and non-bitter cotyledons has the genotype ddrrbb.

100% of the F1 will have dull, orange fruit and bitter cotyledons and will be heterozygous for all genes: DdRrBb.

The F2 will have 8 possible phenotypes:

• 27 D_R_B dull, orange fruit and bitter cotyledons
• 9 D_R_bb dull, orange fruit and non-bitter cotyledons
• 9 D_rrB_ dull, cream fruit and bitter cotyledons
• 3 D_rrbb dull, cream fruit and non-bitter cotyledons
• 9 ddR_B_ glossy, orange fruit and bitter cotyledons
• 3 ddR_bb glossy, orange fruit and non-bitter cotyledons
• 3 ddrrB_ glossy, cream fruit and bitter cotyledons
• 1 ddrrbb glossy, cream fruit and non-bitter cotyledons

b)

An F1 plant is crossed with a plant that has glossy, cream fruit and non-bitter cotyledons.

DdRrBb x ddrrbb

The F1 individual can produce 8 types of gametes: DRB, DRb, DrB, Drb, dRB, dRb, drB and drb. The cream fruit and non-bitter cotyledons individual can only produce drb gametes. This cross also produces a progeny with the following genotypes and phenotypic proportions:

• 8 DdRrBb dull, orange fruit and bitter cotyledons
• 8 DdRrbb dull, orange fruit and non-bitter cotyledons
• 8 DdrrBb dull, cream fruit and bitter cotyledons
• 8 Ddrrbb dull, cream fruit and non-bitter cotyledons
• 8 ddRrBb glossy, orange fruit and bitter cotyledons
• 8 ddRrbb glossy, orange fruit and non-bitter cotyledons
• 8 ddrrBb glossy, cream fruit and bitter cotyledons
• 8 ddrrbb glossy, cream fruit and non-bitter cotyledons